The internal interaction is the first-order quadrupole interaction HQ with xQ ¼ 2pCQ =4Ið2I 1Þ, where CQ = e2Qq=h is a parameter called a quadrupole coupling constant. The term RQ 2;0 represents the spatial orientation of the electric field gradient (EFG) tensor with respect to static magnetic field. In the presence of molecular motion, the tensor orientation may be transformed from the principal axis system (PAS) to the laboratory frame by several steps, which is expressed by